We have been borrowing an older probably ~08 Liquid Force Venture 5'6" Everyone is easily capable of getting up no problems. Almost everyone can ride ropeless and the 1 or 2 who can't can get slack about 90% of the time the board just seems too slow for them to stay in the sweet spot.

Right now we are just using stock ballist on an 07 X-45 with thoughts of getting either a 750 or 1100 sac to add on.

What's a good board to look at that would still be easy enough for the semi-beginner to get to ropeless yet good enough that the guys advancing quicker can start thinking about some fancier riding?

After reading a ton it looks like the Inland Surfer Swallow V2 Quad Fin might be a good fit?

You definitely want to get at least the FH 750 for the locker. I would also get a couple of 400's since you will be adding weight later as you progress. As far as the boards...You will want to post your weights and abilities as this will help the members give you good advice on selection. I would strongly recommend spending the time to go to a demo and riding a few boards before making the investment.

+1 on the v2 Swallow
Wicked fast down the line. 4 fins make it track great, which is important for beginners.
It's my primary board and I weigh 165 lbs. I have friends over 200 lbs (novices) who have ridden it as well.